知道 Redis RDB 这些细节，可以少踩很多坑

2022-06-23 00:00:00 专区订阅付费交易评论

在使用 Redis 的过程中，你是否遇到过下面这些问题：

开启 RDB 落盘，业务频繁出现请求超时。
除了 save 和 bgsave 命令，还有哪些操作会触发 RDB 落盘？
执行了 flushall，发现 flushall 之前写的数据又冒出来了。
重启 Redis 实例之后，发现数据有丢失的情况。

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master 未开启 RDB 的情况，关闭之前需要主动执行 bgsave，否则会导致数据丢失。这也是“","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]},{"text":"重启 Redis 实例之后，发现数据有丢失的情况","marks":[{"type":"fontSize","value":16},{"type":"bold"}]},{"text":"”的原因。","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"zs3X-1655866267151","name":"paragraph","data":{},"nodes":[{"type":"text","id":"xicm-1655866267150","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 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111","marks":[]}]}],"state":{}},{"type":"block","id":"Kgtx-1655888558340","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"rnAU-1655888558339","leaves":[{"text":"OK","marks":[]}]}],"state":{}},{"type":"block","id":"F8S9-1655888558343","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"XPLp-1655888558342","leaves":[{"text":"127.0.0.1:6301> config get save","marks":[]}]}],"state":{}},{"type":"block","id":"4nkT-1655888558345","name":"list-item","data":{"version":1,"listId":"9Mr3-1655888558326","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"SwYc-1655888558344","leaves":[{"text":"\"save\"","marks":[]}]}],"state":{"index":1}},{"type":"block","id":"Q5Jr-1655888558347","name":"list-item","data":{"version":1,"listId":"9Mr3-1655888558326","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"MLRb-1655888558346","leaves":[{"text":"\"1800 1\"","marks":[]}]}],"state":{"index":2}},{"type":"block","id":"JTCf-1655888558349","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"pUfg-1655888558348","leaves":[{"text":"127.0.0.1:6301> config get appendonly","marks":[]}]}],"state":{}},{"type":"block","id":"L445-1655888558351","name":"list-item","data":{"version":1,"listId":"bbcW-1655888558328","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"fwt3-1655888558350","leaves":[{"text":"\"appendonly\"","marks":[]}]}],"state":{"index":1}},{"type":"block","id":"EX80-1655888558353","name":"list-item","data":{"version":1,"listId":"bbcW-1655888558328","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"ty84-1655888558352","leaves":[{"text":"\"no\"","marks":[]}]}],"state":{"index":2}},{"type":"block","id":"ZXq5-1655888557385","name":"paragraph","data":{},"nodes":[{"type":"text","id":"0noT-1655888557383","leaves":[{"text":"127.0.0.1:6301> shutdown","marks":[]}]}],"state":{}},{"type":"block","id":"fAej-1655888568959","name":"paragraph","data":{},"nodes":[{"type":"text","id":"Hzmi-1655888568957","leaves":[{"text":"启动 Redis","marks":[]}]}],"state":{}},{"type":"block","id":"8esc-1655888579617","name":"paragraph","data":{},"nodes":[{"type":"text","id":"qvf3-1655888579616","leaves":[{"text":"127.0.0.1:6301> get ddd","marks":[]}]}],"state":{}},{"type":"block","id":"ak95-1655888636161","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"C3NS-1655888636160","leaves":[{"text":"\"111\"","marks":[]}]}],"state":{}},{"type":"block","id":"KvFR-1655888569423","name":"paragraph","data":{},"nodes":[{"type":"text","id":"b9uE-1655888569422","leaves":[{"text":"","marks":[]}]}],"state":{}},{"type":"block","id":"LpVB-1655888557524","name":"paragraph","data":{},"nodes":[{"type":"text","id":"Cdb3-1655888557522","leaves":[{"text":"这种情况会写 RDB，重启后数据未丢失。","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"O39z-1655808429874","name":"paragraph","data":{},"nodes":[{"type":"text","id":"FuU6-1655808429873","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"9dLG-1655866291685","name":"paragraph","data":{},"nodes":[{"type":"text","id":"mFAL-1655866291683","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"2f56-1655866291839","name":"paragraph","data":{},"nodes":[{"type":"text","id":"a8do-1655866291837","leaves":[{"text":"4.4 AOF 开启，RDB 关闭的情况","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"},{"type":"bold"}]}]}],"state":{}},{"type":"block","id":"rufi-1655866349417","name":"paragraph","data":{},"nodes":[{"type":"text","id":"FhXv-1655866349416","leaves":[{"text":"127.0.0.1:6301> set eee 111","marks":[]}]}],"state":{}},{"type":"block","id":"jRgB-1655889003755","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"jPpv-1655889003754","leaves":[{"text":"OK","marks":[]}]}],"state":{}},{"type":"block","id":"fMKm-1655889003757","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"SrtN-1655889003756","leaves":[{"text":"127.0.0.1:6301> config get save","marks":[]}]}],"state":{}},{"type":"block","id":"RxG8-1655889003759","name":"list-item","data":{"version":1,"listId":"eSo1-1655889003739","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"M1yY-1655889003758","leaves":[{"text":"\"save\"","marks":[]}]}],"state":{"index":1}},{"type":"block","id":"QyCu-1655889003761","name":"list-item","data":{"version":1,"listId":"eSo1-1655889003739","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"5soX-1655889003760","leaves":[{"text":"\"\"","marks":[]}]}],"state":{"index":2}},{"type":"block","id":"Y5nl-1655889003763","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"a72b-1655889003762","leaves":[{"text":"127.0.0.1:6301> config get appendonly","marks":[]}]}],"state":{}},{"type":"block","id":"nOew-1655889003766","name":"list-item","data":{"version":1,"listId":"sETu-1655889003741","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"hCOU-1655889003765","leaves":[{"text":"\"appendonly\"","marks":[]}]}],"state":{"index":1}},{"type":"block","id":"1jb5-1655889003768","name":"list-item","data":{"version":1,"listId":"sETu-1655889003741","listLevel":1,"listType":"ordered"},"nodes":[{"type":"text","id":"vv9B-1655889003767","leaves":[{"text":"\"yes\"","marks":[]}]}],"state":{"index":2}},{"type":"block","id":"bD3W-1655889003770","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"y0mC-1655889003769","leaves":[{"text":"127.0.0.1:6301> shutdown","marks":[]}]}],"state":{}},{"type":"block","id":"3jde-1655889006481","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"h368-1655889006480","leaves":[{"text":"启动 Redis","marks":[]}]}],"state":{}},{"type":"block","id":"tCQC-1655889021894","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"PUXR-1655889021892","leaves":[{"text":"127.0.0.1:6301> get eee","marks":[]}]}],"state":{}},{"type":"block","id":"xoQU-1655889026836","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"cXWE-1655889026835","leaves":[{"text":"\"111\"","marks":[]}]}],"state":{}},{"type":"block","id":"HEeM-1655894063820","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"ffH5-1655894063818","leaves":[{"text":"这种情况尽管不会进行 RDB 落盘，但是因为之前的操作都写入了 AOF，在 Redis 启动时，会加载 AOF 里的数据，因此也会跟关闭之前的数据保持一致。","marks":[]}]}],"state":{}},{"type":"block","id":"7vzB-1655894152756","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"mwZb-1655894152755","leaves":[{"text":"","marks":[]}]}],"state":{}},{"type":"block","id":"YAwN-1655894152917","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"9p6z-1655894152915","leaves":[{"text":"","marks":[]}]}],"state":{}},{"type":"block","id":"7Ttv-1655894153057","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"PTOy-1655894153055","leaves":[{"text":"到这里，文章开始列出的四个问题都应该找到了答案，可能列出的 RDB 细节不一定全，朋友们可以在手机端打开文章跳到下面，点击“发消息”进行交流。","marks":[]}]}],"state":{}},{"type":"block","id":"jxWH-1655894064030","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"iMEJ-1655894064029","leaves":[{"text":"或者加入数据库交流群讨论，入群请添加下方群秘微信，备注“进群”，等待群秘邀你入群。","marks":[]}]}],"state":{}},{"type":"block","id":"uC0W-1655894356010","name":"paragraph","data":{},"nodes":[{"type":"text","id":"Ki9E-1655894356011","leaves":[{"text":"","marks":[]}]}],"state":{}},{"type":"block","id":"T7WM-1655808430036","name":"paragraph","data":{},"nodes":[{"type":"text","id":"HCQf-1655808430034","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"Er30-1655809352544","name":"paragraph","data":{},"nodes":[{"type":"text","id":"taT4-1655809352542","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"GDHK-1655809366432","name":"paragraph","data":{},"nodes":[{"type":"text","id":"kmTC-1655809366430","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"wG4v-1655808436810","name":"paragraph","data":{},"nodes":[{"type":"text","id":"n1ZI-1655808436809","leaves":[{"text":"","marks":[{"type":"color","value":"#333333"},{"type":"backgroundColor","value":"rgb(255, 255, 255)"},{"type":"fontSize","value":16},{"type":"fontFamily","value":"Arial"}]}]}],"state":{}},{"type":"block","id":"NShA-1655792324924","name":"paragraph","data":{},"nodes":[{"type":"text","id":"VAG8-1655792324923","leaves":[{"text":"标题：知道 Redis RDB 这些细节，可以少踩很多坑","marks":[]}]}],"state":{}}]">

带着上面这些问题，我们一起来聊聊 RDB 的一些细节。

触发 RDB 文件创建的命令有两条，save 和 bgsave。

save 我们知道会阻塞整个实例，通常也不太可能会用。

bgsave 命令是在后台生成 RDB 文件，Redis 仍然可以处理客户端请求。

但是并不能保证 bgsave 不会影响 Redis 所有的客户端请求，在生成 RDB的过程中，Redis 会 fork 出一个子进程，子进程和父进程会共享内存地址空间，可以保证子进程拥有父进程相同的内存数据。但是在 fork 子进程时，操作系统需要将父进程的内存页表复制给子进程。如果整个 Redis 实例占用的内存很大，那么它的内存页表也会很大，复制的时间也会比较长。

同时，这个过程会消耗大量的 CPU 资源，在复制完成之前，父进程也会被阻塞，无法处理客户端请求。

执行 fork 后，子进程可以扫描 Redis 中所有数据，然后将所有数据写入 RDB 文件。

之后，父进程仍然处理客户端的请求。父进程在处理写命令时，会重新分配新的内存地址空间，向操作系统申请新的内存使用，不再与子进程共享。这样，父子进程的内存会逐渐分离，父进程会申请新的内存空间并改变内存数据，子进程的内存数据不会受到影响。

可以看出，在生成RDB文件时，不仅消耗CPU资源，还需要消耗更多的内存空间。

上面也就是“开启 RDB 落盘，业务频繁出现请求超时”的原因。通常在生产环境，我们也应该避免在 master 实例上做 RDB。

那么除了 save 和 bgsave 命令，还有哪些常见会触发 RDB 呢？这里就来总结几种情况，这也是问题“除了 save 和 bgsave 命令，还有哪些操作会触发 RDB 落盘呢？”的答案：

1 配置了 RDB 落盘的情况

比如配置文件配置了 save xxx xxx，或者命令行执行了 config set save "xxx xxx"，都表示配置了 RDB 落盘。

比如配置了 save 900 1

则表示 900 秒内如果至少有 1 个 key 的值变化，则做一次 bgsave。

我们在前面有提到 bgsave 也会对客户端请求有所影响，所以不建议在 master 上增加该参数，如果为了数据备份，建议只在 slave 增加 save 参数。

2 主从复制

次创建主从复制关系时，会在主库执行 bgsave 命令生成 RDB 文件，然后传给从库，从库加载 RDB 文件，以完成一次全量数据的传输。

因此在业务访问高峰，并且数据量比较大的情况，不建议在 master 上创建 slave。

3 Redis 在执行 flushall 命令

配置了 RDB 落盘的情况，在执行 flushall 命令时，会进行一次 RDB 落盘，但是内容是空的。目的是将 RDB 文件也清空。

但是，如果 RDB 和 AOF 都关闭的情况下，会有下面这种情况：

127.0.0.1:6301> set aaa 111OK127.0.0.1:6301> set bbb 111OK127.0.0.1:6301> bgsaveBackground saving started127.0.0.1:6301> flushallOK127.0.0.1:6301> config get save"save"""127.0.0.1:6301> config get appendonly"appendonly""no"127.0.0.1:6301> shutdown

再启动 Redis

127.0.0.1:6301> keys *"aaa""bbb"

会看到我们清空 Redis 之前写入的数据。显然是不符合逻辑的。

这是因为在启动 Redis 时，会加载数据目录下的 RDB 文件，而这个 RDB 文件是 flushall 之前执行 bgsave 生成的，也就是会看到清空 Redis 之前写入的数据。这里也是“执行了 flushall，发现 flushall 之前写的数据又冒出来了”的原因。

所以在实例未开启 RDB 和 AOF 的情况下，如果执行了 flushall 命令，建议再执行一次 bgsave，让 RDB 文件也清空。

另外还测试了开启 AOF，关闭 RDB 的情况：

127.0.0.1:6301> set aaa 111OK127.0.0.1:6301> set bbb 111OK127.0.0.1:6301>  bgsaveBackground saving started127.0.0.1:6301> flushallOK127.0.0.1:6301> config get save"save"""127.0.0.1:6301> config get appendonly"appendonly""yes"127.0.0.1:6301> shutdown

启动 Redis

127.0.0.1:6301> keys *(empty list or set)

重启之后不会再看到 flushall 之前写入的数据，因为 Redis 在启动时加载 RDB 文件后，也会加载在执行 RDB 之后 AOF 里新增的操作。而 flushall 操作就记录在 AOF 文件中。

4 正常关闭时

我们通过在命令行执行 shutdown 正常关闭 Redis 时，并不是所有情况都会执行一次 RDB 落盘的，这里就来分析一下不同配置，重启后的情况。

4.1 AOF 和 RDB 都开启的情况

127.0.0.1:6301> set bbb 111OK127.0.0.1:6301> config get save"save""1800 1"127.0.0.1:6301> config get appendonly"appendonly""yes"127.0.0.1:6301> shutdown

启动 Redis

127.0.0.1:6301> get bbb"111"

这种情况会在关闭时执行一次 RDB 落盘，启动时加载 RDB 文件，保证重启前后数据一致。

4.2 AOF 和 RDB 都未开的情况

127.0.0.1:6301> set ccc 111OK127.0.0.1:6301> config get save"save"""127.0.0.1:6301> config get appendonly"appendonly""no"127.0.0.1:6301> shutdown

然后启动 Redis

127.0.0.1:6301> get ccc(nil)

发现重启之前 ccc 的 key 已经丢失，因此在 master 未开启 RDB 的情况，关闭之前需要主动执行 bgsave，否则会导致数据丢失。这也是“重启 Redis 实例之后，发现数据有丢失的情况”的原因。

4.3 AOF 关闭，RDB 开启的情况

127.0.0.1:6301> set ddd 111OK127.0.0.1:6301> config get save"save""1800 1"127.0.0.1:6301> config get appendonly"appendonly""no"127.0.0.1:6301> shutdown

启动 Redis

127.0.0.1:6301> get ddd"111"

这种情况会写 RDB，重启后数据未丢失。

4.4 AOF 开启，RDB 关闭的情况

127.0.0.1:6301> set eee 111OK127.0.0.1:6301> config get save"save"""127.0.0.1:6301> config get appendonly"appendonly""yes"127.0.0.1:6301> shutdown

启动 Redis

127.0.0.1:6301> get eee"111"

这种情况尽管不会进行 RDB 落盘，但是因为之前的操作都写入了 AOF，在 Redis 启动时，会加载 AOF 里的数据，因此也会跟关闭之前的数据保持一致。

到这里，文章开始列出的四个问题都应该找到了答案，可能列出的 RDB 细节不一定全，朋友们可以在手机端打开文章跳到下面，点击“发消息”进行交流。

原文链接：https://mp.weixin.qq.com/s/RiOD50FnmbWfpHzyk5nFdA